In the recovery of crude plant oils from the raw materials, phosphorous-containing compounds, namely phosphoglycerides and phosphosphyngolipids customarily are found in the plant oil. Such substances, referred to generally as gum substances or phosphatides derive from the cells of the raw material and pas into the oil during the oil extraction process.
Such compounds play a role in the life processes of the plant, for example in the formation of the lipoprotein cell membranes, in food synthesis, in fatty-acid metabolism and in other processes which take place within the cells.
The quantities of these substances which can be found in the plant oil fluctuate depending upon the characteristics of the raw plant materials and the technology used for oil recovery. They may make up between 3.0 and 0.5% by weight of the plant oil.
In spite of their presence in relatively small quantities, their composition may be complex. For example, the principal proportion of such gum substances may be constituted from at least 10 to 12 compounds, which is not surprising since such materials have a variety of functions in the cells and hence a multiplicity of compounds can be expected to be present in this phosphatide component.
The multiplicity of compounds present in the phosphatide component means that some of the component will have different properties than others. For example, most of these compounds are hydratable by water. They form lyotropic phases and are swellable so that they can be readily separated in a gel form from the plant oil by water degumming techniques.
The phosphatides also contribute to the cloudiness of the plant oil and precipitate formation. They may disturb further oil refining processing steps and hence removal of them is necessary. The removal of so-called hydratable gums can be effected by a treatment with water or steam, swelling or hydration with subsequent separation, usually by centrifugation. These process steps are referred to generally as aqueous degumming or water degumming.
The gumming substances present in the plant oil, however, also include compounds which are not hydratable in the presence of the water molecule and thus remain in the oil after water degumming.
The amount of so-called nonhydratable gums or gum substances, depending upon the nature of the water degumming process which is carried out, can amount to about 0.15 to 0.20% by weight of the plant oil or between 5 and 30% of the total gum substances originally present. Removal of such nonhydratable gums requires special methods.
It has been found to be important that even these relatively small amounts of nonhydratable gums be reduced in the course of further refining of the oil and deodorizing so that the remaining gum substances are present in a total concentration which is as much as possible reduced below 0.01% to avoid problems in the deodorizing apparatus and with the raffinate quality. This is especially important since the gummy substances are not significantly thermally stable and can undergo polymerization and cracking at the customary deodorizing temperatures which can exceed 200.degree. C. The decomposition products of phosphatides remaining in the raffinate also detrimentally affect the taste of the fully refined oil.
In the classical chemical refining processes in which the free fatty acids are neutralized with alkali and are removed in the form of soaps which can be washed from the oil, the gum substance content can be reduced to about 0.015 to 0.03%. The requisite further reduction can be effected in the bleaching stage before the subsequent deodorization.
Because of the increasing significance of physical refining processes which are being practiced to a greater extent for greater numbers of oils, the degumming is followed by bleaching and a distillative deacidification and deodorization.
The requisite elimination of the remaining gum substances after the water degumming step can thus be achieved, only with increased use of bleaching earth and thus at a significant cost increase. It is, therefore, of great importance to be able to reduce the gum substance content in the initial stages of refining.
There has been considerable research into the nature of these nonhydratable compounds: (K. Nielsen: Dissertation Copenhagen 1956; B. Braae, U. Brimberg and N. Nyman: J. Am. Oil Chem.Soc., 34, 1957, 293; A. Hvolby: Femte Nordiska Fettsymposiet, Tyringe, 1969, 338-351; C. R. Scholfield, H. J. Dutton et al: J. Am. Oil Chem.Soc., 25, 1948, 368-372, etc.).
The most significant conclusions are that in contrast to the hydratable gums, whose phosphatide molecules have a highly polar component, for example, choline, ethanolamine, serine and, inosite, the nonhydratable gums do not have these polar portions and are constituted primarily of the calcium and magnesium salts of the phosphatidic acids and the lysophosphatidic acids. Of course the salt formation can also take place with other cations, for example iron, copper and aluminum.
Based upon their structure, such nonhydratable phosphatides can be removed, according to the literature, by a variety of processes. These processes have been found to be successful for elimination of the majority of the compounds which are nonhydratable with water.
One such process as described in German open application DE-OS 26 09 705 treats the oil with acid or acid anhydride and subsequently with water. In U.S. Pat. No. 4,049,686 the oil is also treated with acid and the oil/acid mixture is subjected to washing out of the acid and 0.5 to 3% water is added to the acid-reacted phosphatide.
As in the more recent European patent publication 0 195 991, the removal by this process of metallic impurities is not satisfactory and the efforts to remove nonhydratable phosphatides to the low levels required involve a variety of difficulties.